The present invention pertains in general to methods for metering two-phase flow and in particular to methods for metering two-phase flow using an orifice plate and a venturi in series.
In an oil field in which steam injection is employed to enhance oil recovery, each of a number of steam injectors may be fed by a branch of a trunk line from a common steam generator. Due to flow-splitting phenomena at the branches, a different ratio of steam to total flow (steam plus water), also called steam quality, is likely to be present in each branch.
A knowledge of the ratio of steam to total flow being injected in a two-phase flow is critical to any understanding of the effects of steam injection. Because it is impractical to predict this ratio from analysis of the injection apparatus, it is important to be able to determine flowrate parameters for calculating steam quality from measurements made at each branch.
Many methods for metering single-phase flow, such as those dependent upon critical choke flow or those employing single orifice meters, lose their accuracy when applied to a two-phase flow system. Other methods, such as steam calorimetry, have inherent sampling problems.
Two-phase flow may be metered by employing two or more measurements which are mathematically correlated.
One such approach involves the use of a gamma ray densitometer to make void fraction measurements and a turbine meter or drag disc to obtain a second measurement. This approach is limited to a small quality range and requires the use of an expensive and delicate gamma ray densitometer instrument.
In another such approach, exemplified by K. Sekoguchi et al, "Two-Phase Flow Measurements with Orifice Couple in Horizontal Pipe Line", Bulletin of the ISME, Vol. 21, No. 162, December, 1978, pp. 1757-64, two segmental orifices or baffles are coupled in series. The pressure drop across each orifice or baffle is measured and correlated with the pressure drop across the other orifice or baffle. The orifices must differ in configuration in order to provide independent measurements for the purpose of correlation. One drawback of this approach is that data is not presented in dimensionless form suitable for predicting performances for different systems.
Yet another such approach involves measurement of a frictional pressure drop across a twisted tape, measurement of an accelerational pressure drop across a venturi and correlation of the results. A disadvantage of this approach is that a very sensitive device is required to measure the pressure drop across the twisted tape.
Measurement of the pressure drops across a venturi and an orifice in series may be done simply and at reasonable cost, as shown in D. Collins et al, "Measurement of Steam Quality in Two-phase Upflow with Venturi Meters and Orifice Plates", Journal of Basic Engineering, Transactions of the ISME, March 1971. Although concurrent pressure drops were measured for calibration purposes in Collins et al, pp. 11-21, the pressure drops across an orifice plate and across a venturi coupled in series have not previously been correlated for the purpose of metering two-phase flow prior to the present invention.